Goss-Custard J.D. (1980) Competition for food and interference among waders. ARDEA 68 (1-4): 31-52

This paper discusses how the rate of food intake of wading birds may decrease as their density increases. It distinguishes two main processes, depletion and interference. Interference is the more-or-less immediate, and reversible, reduction in rate when wader density increases. It occurs because the actual presence of other birds themselves in some way interferes with feeding. In depletion, the reduction in the food supplies themselves reduces feeding rate. Field evidence is reviewed for interference in Haematopus ostralegus, Tringa totanus and Numenius arquata. Although little studied, interference may occur because, as wader density increases, (I) birds fight more for food or feeding sites, (II) kleptoparasitism increases, (III) birds disturb or distract each others' searching more frequently, (IV) the available fraction of prey is depleted more, (V) the prey retreat more into the mud, or (VI) more birds are forced into the poorer feeding areas. The evidence for each of these possibilities is discussed, but few conclusive data have been published to date. Depletion of the food may be considerable during the winter, with waders taking 25-45 per cent of the food present in their main feeding areas. Such reductions could reduce average feeding rate by up to 30 per cent in T. totanus and H. ostralegus. The deleterious effects of interference and depletion could be distributed unequally amongst the population so that some individuals bear the brunt of the decrease. Field evidence suggests that, in most winters, mortality is slight but that the relatively poor feeding conditions at that time of year may be involved: in very cold winters, starvation and heavy mortalities do occur. Since intake rate is depressed by increased population density, a greater proportion of birds are likely to be at risk as population density rises. Though slight, winter mortality linked to poor feeding conditions could thus be density-dependent, although more field studies are required to test this possibility. Using a theoretical population model in which most regulation is through territoriality in the breeding season and the population is subject to a larger density-independent mortality in the summer, it is shown that the introduction of a weakly density-dependent winter mortality of only a few per cent could decrease stable population size by 25-45 per cent. Therefore we should be cautious before dismissing such a mortality as trivia!. The use of models like this in studies of the population dynamics of these migratory birds is discussed. Competition, interference and depletion may be defined in terms of some effect and by the process involved. With the possible exception of kleptoparasitism, competition is probably involved in interference and depletion however terms are defined.